I have felt very much like you. We dealt with it using a two pronged approach. Firstly we negotiated and a good negotiator does not give up on the first come back, you go back again, and again if necessary.
Secondly I decided that I wanted to be the best version of myself that I could be so that Z would want to be with me. I don't believe we can make people love us or want to be with us. All we can do is be the best that we can be and if that isn't enough maybe it is time to walk.
I think it is fairly easy to tell a poly from a player by the amount of work they are prepared to put into your relationship to make it work. You say he is wonderful but it doesn't sound as if he is behaving very wonderfully.